Containers holding liquids and bulk solids are economically manufactured in a continuous blow molding process wherein a parison comprising a hollow tube of molten polymer resin is extruded continuously from a flow head. A series of moving molds act upon the parison. Each of the moving molds is formed of opposing mold halves which sequentially engage a respective portion of the parison by closing about the parison from opposite sides. As the mold halves comprising a particular mold close about a parison portion, knives on the mold halves sever the parison portion from the continuously extruding parison. The mold then moves away from the flow head to allow the next mold to engage its respective parison portion. After a parison portion is engaged by a mold and cut from the parison, air is injected into the parison portion forcing it to expand and assume the shape of the mold. The mold is then opened (horizontally) to release the newly molded container (ejected downwardly with gravity assist) to a conveyor, which transports the container for further processing. The mold then travels back to the flow head to mold the next container. A detailed description of a process and apparatus for producing the containers is provided in U.S. Pat. No. 5,840,349, incorporated in this document by reference in its entirety.
In a method of molding a hollow plastic container in accordance with U.S. Pat. No. 7,153,127 issued to Struble et al., at least three mold segments are closed around a hollow tube of plastic material having a longitudinal axis by moving at least two of the mold segments toward the third mold segment in directions that are non-parallel to the axis of the tube. The mold segments when closed form a mold cavity that surrounds and captures a portion of the tube. A blow needle or pin on one of the mold segments pierces the tube when the mold segments are closed around the tube and the tube is blown through the blow pin to the internal confines of the mold cavity. The mold cavity is then opened and the blown container is removed from the mold. One of the mold segments preferably comprises a mold core movably mounted on a second of the mold segments, and pneumatic cylinders are mounted on the second mold segment for moving the mold core between an extended position and a retracted position with respect to the second mold segment.
As illustrated by the machine disclosed by Struble et al., blow molding machines often include electrical, hydraulic, or pneumatic devices (typically cylinders) to move the mold segments relative to one another. Such devices are undesirable for a number of reasons. They add to the complexity of the machine, thereby increasing the cost of the machine. Electrical, hydraulic, or pneumatic devices also increase maintenance costs and decrease the rate of producing containers. Often, it is desired to stack the containers. U.S. Pat. No. 8,205,749 issued to Korpanty et al. discloses the advantages of a stackable container assembly that is configured to ensure accurate positioning and registration between respective container assemblies when they are stacked. The stackable container assembly includes a main body; an upstanding rim portion having an upper surface; and a flexible bottom portion. The flexible bottom portion has a lower surface comprising at least one support surface for supporting the container assembly on a horizontal surface. The lower surface further comprises at least one projection (surrounded by a recess) that is disposed radially inwardly from the support surface, and wherein the flexible bottom portion is configured so that the projection will flex downwardly into a position that is adjacent to an upstanding rim portion of an underlying container assembly when the container assembly is stacked on top of another container assembly.
Korpanty et al. address the need for an improved plastic container that is configured so as to ensure accurate registration during stacking. Others have addressed that same need using a variety of bottom container configurations. As discussed below, however, such bottom container configurations create challenges during the molding process, including, for example, the need to avoid damage to projections, corners, and other features of the container bottom as the mold halves open.
Often, as described above, the typical blow molding machine used to form containers has molds configured to form a single bottle at a time. In another exemplary extrusion blow molding operation, pairs of plastic bottles (i.e., containers) are manufactured simultaneously by blow-molding a two-bottle log. Each bottle container portion is formed on an end of the log. The bottles are formed neck-to-neck, with the bottle necks facing each other and joined by a neck ring. The neck ring must be trimmed away to separate the bottles. U.S. Pat. No. 7,752,947 issued to Fiorani et al. discloses a bottle trimmer and method of cutting or trimming a plastic log ejected from a blow mold to form multiple open-mouthed bottles. FIGS. 1 and 2 show a two-bottle log A as ejected from a blow molding machine. The log A includes two blow molded bottles B having necks C located on an axis D offset to one side of the sides of the bottles. Neck ring E extends between the two necks C. Neck flash F extends in the recess between the bottles to one side of the necks and ring.
FIG. 3 illustrates log A with neck flash F and G trimmed away. FIG. 4 illustrates the log with neck ring E trimmed away to separate bottles B and complete the trimming operation. The trimmer and method are especially suitable for high-output production lines. Each of the bottles B has a bottom I which is curved, or has an undercut, so that the bottom I is not flat. This configuration of the bottom I creates a lip or corner around the edge of the bottom I. The curved bottom I is often required for a container adapted to be used with products that create relative pressure changes between the interior of the container and ambient conditions. One example of such a product is yogurt, in which fermentation produces carbon dioxide by yeast activity and increases pressure inside the yogurt container. More generally, fermentation in food processing is the conversion of carbohydrates to alcohols and carbon dioxide or organic acids using yeasts, bacteria, or a combination of the two under anaerobic conditions.
The containers must be designed so that they can be removed from the mold after blowing. This requires that any undercuts must be shallow enough to permit the container to be stripped from the mold when the mold is opened. This limits, in turn, the push-up height that can be achieved, and requires that the cross section of the container have a positive draft toward the mold parting line.
A need exists for an improved plastic container assembly that is configured so as to ensure accurate registration during stacking and so as to minimize unwanted container distortion that might otherwise occur as a result of relative pressure changes between the interior of the container and ambient conditions. Provision of a mold having mold segments that are mounted on, and mechanically movable relative to, another mold segment allows molding of plastic containers having contours, features, and geometries that have not been blow-moldable to date, including containers having deep undercuts, containers having elaborate cross sections, and containers with recesses that facilitate stacking and storing. Therefore, a general object of the present invention is to provide a method and apparatus for molding hollow plastic containers having improved versatility in terms of the geometries, features, and contours of the containers that can be molded.
Although known continuous blow molding machines allow for high production rates of uniform containers, there are disadvantages in the various machines which, if eliminated, would result in more reliable production of high quality containers. One such problem involves the mechanics of opening and closing the mold, which affects the quality of the molded container. It is important that the molds close in precise alignment consistently and maintain the precise alignment throughout the molding process. The molds must withstand significant internal pressure without shifting or parting to ensure a quality container with the requisite uniformity of production.
To overcome the shortcomings of conventional apparatus, an extrusion blow molding machine having molds that are moved mechanically without the aid of electrical, hydraulic, or pneumatic assistance is provided. Thus, an object of the present invention is to avoid electrical, hydraulic, or pneumatic devices. A related object is to provide an improved blow molding machine of reduced complexity, weight, and cost. Another related object is to minimize the costs required to maintain the machine.